1. Field of the Invention
Exemplary embodiments of the present invention generally relate to an image forming apparatus, and more particularly, to an image forming apparatus that includes a collection mechanism for recycling toner remaining on an image transfer member after image transfer.
2. Discussion of the Related Art
Related-art image forming apparatuses such as copiers, printers, facsimile machines, and printing presses generally include a developing unit, which develops an electrostatic latent image formed on a photoconductor or an image carrier into a visible image, and a transfer unit that transfers the visible image onto a sheet-like recording medium so as to output an image.
Such image forming apparatuses include either a single photoconductor or multiple photoconductors.
In addition to production of monochrome images, the single photoconductor can output a full-color image by sequentially forming visible images developed with respective toners of complementary colors using a color separation technique and transferring the images onto a recording medium either directly or via an intermediate transfer member.
By contrast, the multiple photoconductors each form respective single color images that are then transferred onto the intermediate transfer member to form an overlaid toner image. The multiple photoconductors may be incorporated in separate respective image forming units and disposed along the intermediate transfer member, the configuration of which is called a tandem-type configuration.
The tandem-type image forming apparatus consumes black toner frequently in the course of producing full-color images, monochrome images, and pictorial full-color images, or for the purpose of color tone adjustment. Therefore, as part of a recent trend of conserving resources, residual toner remaining on an image carrier after image transfer is collected to re-use for development as recycled toner. Recycled toner can be used in both monochrome image forming and full-color image forming operations.
To accomplish such recycling, in a tandem-type image forming apparatus that incorporates separate image carriers corresponding to each toner color, a cleaning unit is provided for removing toner from each image carrier. With this configuration, it is easy to collect and return the residual toner to a developing unit of each toner color for recycling for image formation.
Since the respective image forming units are independent of each other, in theory the tandem-type image forming apparatus as described above should have no mixing of toner colors in the collected toner. However, mixing of toner colors can occur during the image transfer process, in which a toner image is transferred from an image carrier onto a recording medium.
There are several reasons for such mixing of toner of different colors. For example, when forming an overlaid color toner image in the tandem-type image forming apparatus, a first toner image formed on a first image carrier is transferred onto a recording medium conveyed by a sheet transfer member, then a second toner image formed on a second image carrier disposed downstream from the first image carrier is overlaid on the first toner image on the recording medium, and this operation is repeated until a toner image formed on an image carrier farthest downstream is transferred onto the recording medium.
When the downstream-side toner image is transferred onto the recording medium, the toner on the upstream-side toner image that is already carried on the recording medium can reversely be transferred onto the surface of the downstream-side image carrier and is then collected by a cleaning unit for the downstream-side toner. Thus, the upstream-side toner and the downstream-side toner are mixed together. In other words, the first toner carried by a recording medium is reversely transferred onto a second image carrier and is collected by a second cleaning unit. The mixing of toners can also occur in the related-art image forming apparatus with the intermediate transfer system including the intermediate transfer member.
Consequently, when the related-art color image forming apparatus causes toner of each image carrier to be collected and returned to a corresponding developing unit for the purpose of recycling, hue of toner in the developing unit gradually but largely changes, becoming increasingly mixed with time.
Various techniques for known image forming apparatuses have been proposed to eliminate the above-described problem.
In one example of a known image forming apparatus, a black image carrier is located at an extreme upstream side or a first position in an order of image transfer, so that black toner collected from the black image carrier does not get mixed with other colors and can be conveyed to a corresponding developing unit for recycling.
In another example of a known image forming apparatus, toner of mixed color is mixed with black toner to be used as black toner.
Yet another example of a known image forming apparatus has a configuration in which collected toner can be selectively used or entirely discarded.
Further, yet another example of a known image forming apparatus includes a developing unit only for mixed toner.
At present, image forming apparatuses capable of producing color images currently on the market are used at a rate of 70% to 80% to produce monochrome (black-and-white) images. Since black toner is consumed when producing full-color image as well as monochrome image, a relatively large proportion of waste toner consists of black toner. Therefore, even when color toners other than black toner are discarded while the black toner is collected for reuse, in effect substantially all collected toner is not discarded but is practically reused.
As an example, FIG. 1 illustrates a schematic configuration of an image forming mechanism of a generally known image forming apparatus 100. The known image forming apparatus 100 includes image carriers 102K, 102Y, 102M, and 102C and an intermediate transfer belt 115. The image carriers 102K, 102Y, 102M, and 102C are aligned horizontally and rotate in directions identical to each other as indicated by arrows A shown in FIG. 1.
An outer surface of the intermediate transfer belt 115 that forms an endless loop and is extended by supporting rollers contacts the image carriers 102K, 102Y, 102M, and 102C and rotates with the image carriers 102K, 102Y, 102M, and 102C in a direction indicated by arrows B in FIG. 1. In the image forming apparatus 100 of FIG. 1, the image carrier 102K forms monochrome or black (K) images, the image carrier 102Y forms yellow (Y) images, the image carrier 102M forms magenta (M) images, and the image carrier 102C forms cyan (C) images.
The image carrier 102K for forming black images, together with a corresponding developing unit, not shown, are disposed at a far upstream side in the direction B of rotation of the intermediate transfer belt 115, and therefore of all the image carriers are located farthest from a transfer position S where a toner image formed on the outer surface of the intermediate transfer belt 115 is transferred onto a transfer sheet as a recording medium.
Because of the above-described location of the image carrier 102K within the array of image carriers, even though formation of monochrome or black images is the most common of all image forming operations of apparatuses currently on the market, additional time is needed from development of the black image to transfer of the black image onto a recording medium or a transfer sheet. Therefore, not only does a user have to wait from execution of a printing request to completion of printout of the transfer sheet, but also the image forming carriers 102K, 102Y, 102M, and 102C of the image forming apparatus 100 are required to keep idling. In particular, the idling of units unnecessary for forming black images, i.e., the image forming carriers 102Y, 102M, and 102C, of the image forming apparatus 100 can accelerate wear of parts and components used during the idling of the image forming carriers 102Y, 102M, and 102C and shorten the useful life period of the parts and components.
In another related-art image forming apparatus, the mixing of toner of different colors in the collected toner is controlled to remain at or below a predetermined level for recycling in a developing unit for black toner. However, the mixed toner can change the color tone of black toner and degrade the quality of a printed image.
In yet another related-art image forming apparatus, whether to reuse or discard the collected toner is determined by switching a direction of rotation of toner conveyance screws disposed within the individual units. Since a cleaning unit of the related-art image forming apparatus contains mixed toner, even if an amount of toner used for forming each image is obtained by counting the pixels, the calculation actually results in an integral value, making it difficult to accurately estimate the degree of mixed toner in the collected toner.
In some other known image forming apparatuses, a dedicated developing unit dedicated to mixed toner and a dedicated image carrier are incorporated. Consequently, the number of units increases, which can lead to an increase in size of the known image forming apparatus and an increase in manufacturing costs of the known image forming apparatus.
To eliminate such problems, a related-art image forming apparatus 200 shown in FIG. 2 includes four image forming units 220Y, 220M, 220C, and 220K including image carriers 202Y, 202M, 202C, and 202K, and an intermediate transfer belt 215. The image carrier 202Y forms yellow (Y) images, the image carrier 202M forms magenta (M) images, the image carrier 202C forms cyan (C) images, and the image carrier 202K forms black (K) images. The related-art image forming apparatus 200 shown in FIG. 2 employs a tandem-type system. In this case, in the tandem-type image forming apparatus 200, the image forming units 220Y, 220M, and 220C are disposed along the intermediate transfer belt 215 in contact with an outer surface of the intermediate transfer belt 215. The image forming unit 220K of the related-art image forming apparatus 200 is separated from the image forming units 220Y, 220M, and 220C and is located upstream from the image forming units 220Y, 220M, and 220C in a direction of conveyance of a transfer sheet or recording medium. That is, the image forming unit 220K of the related-art image forming apparatus 200 can transfer a black image formed thereon onto the transfer sheet before a composite color image that includes colors other than black is transferred onto the transfer sheet.
As described above, the related-art image forming apparatus 200 shown in FIG. 2 can transfer the black image onto the transfer sheet before the composite color image of yellow, magenta, and cyan images to the transfer sheet. With this configuration, when only a black image or monochrome image is required to be output, the black image transferred onto a black image transfer unit can be electrostatically attracted to the transfer sheet at a position facing a sheet conveyance unit. Therefore, the black or monochrome image can be transferred onto the transfer sheet more quickly than the composite color image, that is, the transfer of black or monochrome images can reduce the operating time.
However, unlike the image forming units 220Y, 220M, and 220C that are held in contact with the intermediate transfer belt 215, the image forming unit 220K includes the black image transfer unit. The black image transfer unit receives the toner image from the image carrier 202K and contacts the transfer sheet directly to transfer the black image onto the transfer sheet, one consequence of which is that the black image transfer unit can acquire more paper dust on the surface thereof than the other units. Particularly when transfer sheets having a width smaller than a maximum passable transfer sheet width of each image carrier are conveyed frequently, paper dust from the transfer sheets can accumulate at the same position on a cleaning blade of a cleaning unit of each image carrier. Examples of such transfer sheets are recycled paper, medium quality paper, paper with additives, and the like.
As a result, when residual toner after image transfer is returned to a developing unit disposed in the image unit 220K for black toner image, paper dust can also be collected to accumulate with the residual toner. The accumulation of paper dust in the developing unit can cause the following problems.
When being mixed with the residual toner, the paper dust can easily accumulated in the developing unit, specifically between a developer carrier and a doctor blade that regulates a thickness or height of developer on the surface of the developer carrier. Blockage of the developer at a portion where paper dust is accumulated can cause a decrease in amount of attraction of the developer, which can result in defective images such as images with white streaks.
Further, when paper dust blocked by the doctor blade is released to flow therethrough, the paper dust with the developer adhering to the surface of the transfer sheet can contaminate the transfer sheet, which may result in image degradation.
To avoid the above-described problems, the developing unit provided in the image forming unit 220K needs to be replaced before those of the image forming units 220Y, 220M, and 220C. This requirement can result in an increase in running cost of the image forming apparatus 200.
Moreover, unlike the image forming units 220Y, 220M, and 220C in which the composite color image is transferred onto the intermediate transfer belt 215 and then onto the transfer sheet, the black image formed by the image forming unit 220K is transferred from the black image transfer unit in the image forming unit 220K directly onto the transfer sheet. Therefore, the image quality ultimately depends on the surface properties of each transfer sheet.
As might be expected, the properties of transfer sheets can vary considerably. For example, one transfer sheet such as embossed paper has a smooth but uneven surface with convex and concave portions whereas another transfer sheet such as coarse paper has an unsmooth surface.
The surface properties of the transfer sheet affect to the transfer electric field that moves the toner from the units to ultimately the recording medium. In particular, fluctuations in the transfer electric field can affect transfer efficiency, such that an image transferred onto the transfer sheet can be a defective image of uneven density, for example.
An additional complicating factor is that an intermediate transfer unit includes an elastic layer on the surface of a sheet transfer member used in the intermediate transfer unit to enhance contact of the transfer sheet with the surface of the above-described sheet transfer member. Some materials that are used for the elastic layer can hinder formation of transfer electric field by a direct contact.
As described above, the image forming unit 220K includes the black image transfer unit that directly contact the transfer sheet. Due to differences in the surface properties of the transfer sheets that contact the black image transfer unit of the image forming unit 220K, the transferability of black images formed by the image forming unit 220K can become inferior to that of the composite color image formed by the image forming units 220Y, 220M, and 220C. Given the high rates of utilization of the image forming unit 220K including the black image transfer unit that is typical of image forming apparatuses currently on the market, such loss of transferability is particularly to be avoided.